A known acoustic radio frequency transponder system produces individualized responses to an interrogation signal. The code space for these devices may be, for example, 2.sup.12 codes, or more, allowing a large number of separate transponders to be produced without code reuse. These devices provide a piezoelectric substrate on which an aluminum pattern is formed, for example by a typical microphotolithography process, with a minimum feature size of, for example, one micron. As discussed more fully below, the metallized pattern defines a set of antenna pickups, electroacoustic transducers, acoustic paths, and elements which interact with the acoustic wave to produce an encoding of an acoustic wave which is converted to an electromagnetic wave and retransmitted.
The transponder devices thus include an acoustic wave device, generally operating with surface acoustic waves, in which an identification code is provided as a characteristic time-domain reflection, attenuation, phase delay, and/or transducer interaction pattern in a retransmitted signal, in a system which generally requires that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is distinguished from the emitted signal. In such a device, received RF energy, possibly with harmonic conversion, is emitted onto a piezoelectric substrate as an acoustic wave with an interdigital electrode system, from whence it travels through the substrate, interacting with reflector elements in the path of the wave. A portion of the acoustic wave is ultimately received by the same or a separate interdigital electrode system and retransmitted. These devices typically do not require a semiconductor memory. The propagation velocity of an acoustic wave in a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, assuming that the time for transmission between the radio frequency interrogation system is short as compared to the acoustic delay, the interrogation frequency typically changes by an amount sufficient such that a return signal having a minimum delay may be distinguished from the simultaneously transmitted interrogation signal, and in a manner that the interrogation frequency does not return to the same frequency during a period longer than the maximum acoustic delay period. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference.
Because the encoded information normally includes an identification code which is unique or quasi-unique to each transponder, and because the transponders of this type are relatively light weight and small and may be easily attached to other objects to be identified, the transponders are sometimes referred to as "labels" or "tags". The entire system, including the interrogator/receiver apparatus and one or more transponders, which may be active or passive, is therefore often referred to as a "passive interrogator label system" or "PILS". Other types of passive interrogator label systems are disclosed, for example, in the U.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217, incorporated herein by reference.
When an acoustic wave pulse is reconverted into an electrical signal, it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. This energy is received at a receiver and decoder, typically at the same location as the interrogating transmitter, and the information contained in this response to an interrogation signal is decoded. The tag typically has but a single antenna, used for both receiving the interrogation pulse and emitting an information bearing signal, but known systems employ multiple antennas.
In systems of this general type, the information code associated with and which identifies the passive transponder is built into the transponder at the time that a layer of metallization is finally defined on the substrate of piezoelectric material. This metallization thus defines the antenna coupling, launch transducers, acoustic pathways and information code elements, e.g., reflectors. Thus, the information code in this case is non-volatile and permanent. Since the transponder typically remains static over time, the encoded information may be retrieved by a single interrogation cycle. The information is present in the return signal as a set of characteristic perturbations of the interrogation signal, such as delay pattern and attenuation. In the case of a tag having launch transducers and a variable pattern of reflective elements, the number of possible codes is N.times.2.sup.M where N is the number of acoustic waves launched by the transducers and M is the number of reflective element positions for each transducer. Thus, with four launch transducers each emitting two acoustic waves, and a potential set of eight variable reflective elements in each acoustic path, the number of differently coded transducers is 2048. Therefore, for a large number of potential codes, it is necessary to provide a large number of launch transducers and/or a large number of reflective elements. However, efficiency is lost with increasing complexity, and a large number of distinct acoustic waves reduces the signal strength of the signal encoding the information in each.
The transponder tag thus includes a multiplicity of "signal conditioning elements", i.e., delay elements, reflectors, and/or amplitude modulators, are coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the first signal. Even where the signal is split into multiple portions, it is advantageous to reradiate the multiple portions of the signal through a single antenna. Therefore, a single "signal combining element" coupled to all of the signal conditioning elements and/or signal portions is provided for combining the intermediate signals to produce the second signal. This second signal is coupled out to the same or a separate antenna for transmission as a reply signal. As described above, the signal delay elements and/or the signal combining element impart a known, unique informational code to the second signal.
In the acoustic wave tags described above, the interrogator transmits a first signal that successively assumes a plurality of frequency values within a prescribed frequency range. This frequency range or band may, for example, be about 905-925 MHz, referred to herein as the 915 MHz band. Traditionally, the 915 MHz band has been available for unlicensed use, however, other or conflicting uses of this band may make other bands more attractive in the future.
Preferably, the passive acoustic wave transponder tag includes at least one element having predetermined characteristics, which assists in synchronizing the receiver and allows for temperature compensation of the system. As the temperature rises, the piezoelectric substrate may expand and contract, altering the characteristic delays and other parameters of the tag. Variations in the transponder response due to changes in temperature thus result, in part, from the thermal expansion of the substrate material. Although propagation distances are small, an increase in temperature of only 20.degree. C. can produce an increase in propagation time by the period of one entire cycle at a transponder frequency of about 915 MHz. The acoustic wave is often a surface acoustic wave, although bulk acoustic wave devices may also be constructed.
The transponder is constructed such that i.sup.th delay time t.sub.i =T.sub.0 +K.DELTA.T+.DELTA.V.sub.i, where K is a proportionality constant, .DELTA.T is the nominal, known difference in delay time between the intermediate signals of two particular successive ones of the signal delay elements in the group, and .DELTA.V.sub.i, is a modification factor due to inter-transponder variations, such as manufacturing variations. By measuring the quantities .DELTA.T and .DELTA.V.sub.i, it is possible to determine the expected delay time t.sub.i -T.sub.0 for each and every signal delay element from the known quantities K, .DELTA.T and .DELTA.V.sub.i. The manufacturing variations .DELTA.V.sub.i are comprised of a "mask" variation .DELTA.M.sub.i due to possible imperfections in the photolithographic mask; an "offset" variation .DELTA.O.sub.i which arises from the manufacturing process used to deposit the metal layer on the piezoelectric substrate; and a random variation .DELTA.R.sub.i which is completely unpredictable but usually neglectably small. Specific techniques are available for determining and compensating both the mask variations .DELTA.M.sub.i and the offset variations .DELTA.O.sub.i.
This surface acoustic wave transponder system provides a number of advantages, including high signal-to-noise performance, and the fact that the output of the signal mixer--namely, the signal which contains the difference frequencies of the first (interrogating) signal and the second (reply) signal--may be transmitted over inexpensive, shielded, twisted-pair wires because these frequencies are, for example, typically in the audio range. Furthermore, since the audio signal is not greatly attenuated or dispersed when transmitted over relatively long distances, the signal-processor may be located at a remote location from the signal mixer, or provided as a central processing site for multiple interrogator antennae.
Passive transponder encoding schemes include control over interrogation signal transfer function H(s) and delay functions f(z). These functions therefore typically generate a return signal in the same band as the interrogation signal. Since the return signal is mixed with the interrogation signal, the difference between the two will generally define the information signal, along with possible interference and noise. By controlling the rate of change of the interrogation signal frequency with respect to a maximum round trip propagation delay, including internal delay, as well as possible Doppler shift, the maximum bandwidth of the demodulated signal may be controlled.
Transponders which include an identification based on elements which alter a response of a SAW device are susceptible to interference which reduces the signal-to-noise ratio of the reply signal that is transmitted by the transponder antenna back to the interrogator. Interference may be caused by reflections from various metallized elements disposed on the surface of the SAW device in the acoustic wave travel paths. The amplitudes of such reflections are directly proportional to the change in velocity, .DELTA.V, of a surface acoustic wave as it passes from a metallization-free surface area on the SAW device to a metallized surface area and vice versa. Such surface acoustic wave reflections are reconverted into electrical signals by the transducers in their paths of travel, resulting in spurious electrical signals that appear as noise in the transmitted reply signal.
A known surface acoustic wave passive interrogator label system, as described, for example, in U.S. Pat. Nos. 4,734698; 4,737,790; 4,703,327; and 4,951,057, and shown in FIG. 1, includes an interrogator comprising a voltage controlled oscillator 10 which produces a first signal S1 at a radio frequency determined by a control voltage V supplied by a control unit 12. This signal S1 is amplified by a power amplifier 14 and applied to an antenna 16 for transmission to a transponder 20.
The signal S1 is received at the antenna 18 of the transponder 20 and passed to a signal transforming element 22. This signal transformer converts the first (interrogation) signal S1 into a second (reply) signal S2, encoded with an information pattern. The information pattern is encoded as a series of elements having characteristic delay periods T.sub.0 and .DELTA.T.sub.1, .DELTA.T.sub.2, . . . .DELTA.T.sub.N. Two common types of systems exist. In a first, the delay periods correspond to physical delays in the propagation of the acoustic signal. After passing each successive delay, a portion of the signal I.sub.0, I.sub.1, I.sub.2 . . . I.sub.N is tapped off and supplied to a summing element. The resulting signal S2, which is the sum of the intermediate signals I.sub.0. . . I.sub.N, is fed back to a transponder tag antenna, which may be the same or different than the antenna which received the interrogation signal, for transmission to the interrogator/receiver antenna. In a second system, the delay periods correspond to the positions of reflective elements, which reflect portions of the acoustic wave back to the launch transducer, where they are converted back to an electrical signal and emitted by the transponder tag antenna.
The signal S2 is passed either to the same antenna 18 or to a different antenna 24 for transmission back to the interrogator/receiver apparatus. This second signal S2 carries encoded information which, at a minimum, identifies the particular transponder 20.
The signal S2 is picked up by a receiving antenna 26. Both this second signal S2 and the first signal S1 (or respective signals derived from these two signals) are applied to a mixer (four quadrant multiplier) 30 to produce a third signal S3 containing frequencies which include both the sums and the differences of the frequencies contained in the signals S1 and S2. The signal S3 is passed to a signal processor 32 which determines the amplitude a, and the respective phase .phi..sub.i of each frequency component .phi..sub.i among a set of frequency components (.phi..sub.0,.phi..sub.1,.phi..sub.2 . . . ) in the signal S3. Each phase .phi..sub.1 is determined with respect to the phase .phi..sub.0 =0 of the lowest frequency component .phi..sub.0. The signal S3 may be intermittently supplied to the mixer by means of a switch.
The information determined by the signal processor 32 is passed to a computer system comprising, among other elements, a random access memory (RAM) 34 and a microprocessor 36. This computer system analyzes the frequency, amplitude and phase information and makes decisions based upon this information. For example, the computer system may determine the identification number of the interrogated transponder 20. This I.D number and/or other decoded information is made available at an output 38.
The transponder serves as a signal transforming element 22, which comprises N+1 signal conditioning elements 40 and a signal combining element 42. The signal conditioning elements 40 are selectively provided to impart a different response code for different transponders, and which may involve separate intermediate signals I.sub.0,I.sub.1 . . . I.sub.N within the transponder. Each signal conditioning element 40 comprises a known delay T.sub.i and a known amplitude modification A.sub.i (either attenuation or amplification). The respective delay T.sub.i and amplitude modification A.sub.i may be functions of the frequency of the received signal S1, or they may provide a constant delay and constant amplitude modification, respectively, independent of frequency. The time delay and amplitude modification may also have differing dependency on frequency. The order of the delay and amplitude modification elements may be reversed; that is, the amplitude modification elements A.sub.i may precede the delay elements T.sub.i. Amplitude modification A.sub.i can also occur within the path T.sub.i.
The signals are combined in combining element 42 which combines these intermediate signals (e.g., by addition, multiplication or the like) to form the reply signal S2 and the combined signal emitted by the antenna 18.
In one embodiment, as shown in FIG. 2, the voltage controlled oscillator 10 is controlled to produce a sinusoidal RF signal with a frequency that is swept in 128 equal discrete steps from 905 MHz to 925 MHz. Each frequency step is maintained for a period of 125 microseconds so that the entire frequency sweep is carried out in 16 milliseconds. Thereafter, the frequency is dropped back to 905 MHz in a relaxation period of 0.67 milliseconds. The stepwise frequency sweep 46 shown in FIG. 3B thus approximates the linear sweep 44 shown in FIG. 3A.
Assuming that the stepwise frequency sweep 44 approximates an average, linear frequency sweep or "chirp" 47, FIG. 3B illustrates how the transponder 20, with its known, discrete time delays T.sub.0,T.sub.1 . . . T.sub.N produces the second (reply) signal 52 with distinct differences in frequency from the first (interrogation) signal 51. Assuming a round-trip, radiation transmission time of t.sub.0, the total round-trip times between the moment of transmission of the first signal and the moments of reply of the second signal will be to t.sub.0 +T.sub.0,t.sub.0 +T.sub.1, . . . t.sub.0 +T.sub.N, for the delays T.sub.ON, T . . . , T.sub.1 respectively. Considering only the transponder delay T.sub.N, at the time t.sub.R when the second (reply) signal is received at the antenna 26, the frequency 48 of this second signal will be .DELTA.f.sub.N less than the instantaneous frequency 47 of the first signal S1 transmitted by the antenna 16. Thus, if the first and second signals are mixed or "homodyned", this frequency difference .DELTA.f.sub.N will appear in the third signal as a beat frequency. Understandably, other beat frequencies will also result from the other delayed frequency spectra 49 resulting from the time delays T.sub.0, T.sub.1 . . . T.sub.N-1. Thus, in the case of a "chirp" waveform, the difference between the emitted and received waveform will generally be constant. In mathematical terms, we assume that the phase of a transmitted interrogation signal is .phi.=2.pi.f.tau., where .tau. is the round-trip transmission time delay. For a ramped frequency df/dt or f, we have: 2.pi.f.tau.=d.phi./dt=.omega.. .omega., the beat frequency, is thus determined by .tau. for a given ramped frequency or chirp f. In this case, the signal S3 may be analyzed by determining a frequency content of the S3 signal, for example by applying it to sixteen bandpass filters, each tuned to a different frequency, f.sub.0,f.sub.1 . . . f.sub.E, f.sub.F. The signal processor determines the amplitude and phase of the signals that pass through these respective filters. These amplitudes and phases contain the code or "signature" of the particular signal transformer 22 of the interrogated transponder 20. This signature may be analyzed and decoded in known manner.
In one embodiment of a passive transponder, shown in FIGS. 6 and 7, the internal circuit operates to convert the received signal S1 into an acoustic wave and then to reconvert the acoustic energy back into an electrical signal S2 for transmission via a dipole antenna 70, connected to, and arranged adjacent a SAW device made of a substrate 72. More particularly, the signal transforming element of the transponder includes a substrate 72 of piezoelectric material such as a lithium niobate (LiNbO.sub.3) crystal, which has a free surface acoustic wave propagation velocity of about 3488 meters/second. On the surface of this substrate is deposited a layer of metal, such as aluminum, forming a pattern which includes transducers and delay/reflective elements.
One transducer embodiment includes a pattern consisting of two bus bars 74 and 76 connected to the dipole antenna 70, a "launch" transducer 78 and a plurality of "tap" transducers 80. The bars 74 and 76 thus define a path of travel 82 for a surface acoustic wave which is generated by the launch transducer and propagates substantially linearly, reaching the tap transducers each in turn. The tap transducers convert the surface acoustic wave back into electrical energy which is collected and therefore summed by the bus bars 74 and 76. This electrical energy then activates the dipole antenna 70 and is converted into electromagnetic radiation for transmission as the signal S2.
The tap transducers 80 are provided at equally spaced intervals along the surface acoustic wave path 82, as shown in FIG. 6, and an informational code associated with the transponder is imparted by providing a selected number of "delay pads" 84 between the tap transducers. These delay pads, which are shown in detail in FIG. 7, are preferably made of the same material as, and deposited with, the bus bars 74, 76 and the transducers 78, 80. Each delay pad has a width sufficient to delay the propagation of the surface acoustic wave from one tap transducer 80 to the next by one quarter cycle or 90.degree. with respect to an undelayed wave at the frequency of operation (in the 915 MHz band). By providing locations for three delay pads between successive tap transducers, the phase f of the surface acoustic wave received by a tap transducer may be controlled to provide four phase possibilities, zero pads=0.degree.; one pad=90.degree.; two pads=180.degree.; and three pads=270.degree..
The phase information .phi..sub.0 (the phase of the signal picked up by the first tap transducer in line), and .phi..sub.1,.phi..sub.2 . . . .phi..sub.N (the phases of the signals picked up by the successive tap transducers) is supplied to the combiner (summer) which, for example, comprises the bus bars 74 and 76. This phase information, which is transmitted as the signal S2 by the antenna 70, contains the informational code of the transponder.
As shown in FIG. 7, the three delay pads 84 between two tap transducers 80 are each of such a width L as to each provide a phase delay of 90.degree. in the propagation of an acoustic wave from one tap transducer to the next as compared to the phase in the absence of such a delay pad. This width L is dependent upon the material of both the substrate and the delay pad itself as well as upon the thickness of the delay pad and the wavelength of the surface acoustic wave.
The transducers are typically fabricated by an initial metallization of the substrate with a generic encoding, i.e., a set of reflectors or delay elements which may be further modified by removal of metal to yield the customized transponders. Thus, in the case of delay pads, three pads are provided between each set of transducers or taps, some of which may be later removed. Where the code space is large, the substrates may be partially encoded, for example with higher order code elements, so that only the lower order code elements need by modified in a second operation.
While a system of the type described above operates satisfactorily when the number of tap transducers does not exceed eight, the signal to noise ratio in the transponder reply signal is reduced as the number of tap transducers increases. This is because the tap transducers additionally act as launch transducers as well as partial reflectors of the surface acoustic wave so that an increase in the number of tap transducers results in a corresponding increase in spurious signals in the transponder replies. This limitation on the number of tap transducers places a limitation on the length of the informational code imparted in the transponder replies.
Spurious signals as well as insertion losses may be reduced in a passive transponder so that the informational code may be increased in size to any desired length, by providing one or more surface acoustic wave reflectors on the piezoelectric substrate in the path of travel of the surface acoustic wave, to reflect the acoustic waves back toward a transducer for reconversion into an electric signal.
A transducer 86 may thus be employed in conjunction with reflectors 88 and 90 in a unique configuration which replaces the aforementioned arrangement having a launch transducer 78 and tap transducers 80. In particular, the transducer 86 is constructed to convert electrical energy received at the terminals 92 and 94 into surface acoustic wave energy which propagates outward in opposite directions indicated by the arrows 96 and 98. The launch transducer is constructed in a well known manner with an inter-digital electrode assembly formed of individual electrode fingers arranged between and connected to the two bus bars 100 and 102. In the illustrated pattern, half the fingers are connected to the bus bar 100 and the other half are connected to the bus bar 102. Each electrode is connected to one or the other bus bar and extends toward a free end in the direction of the other bus bar. The distance between the centers of successive fingers is equal to 3.lambda./4 where .lambda. is the center wavelength of the surface acoustic wave. Furthermore, as may be seen, the length of the active region between the ends of the electrodes connected to the bus bar 100 and the ends of the electrodes connected to the bus bar 102 is K.lambda., where K is a proportionality constant.
Surface acoustic waves may encounter frequency selective filtering structures, partial reflectors, fill reflectors, phase delay pads or electroacoustic transducing elements as they travel across the substrate, which is typically lithium niobate (LiNbO.sub.3) which has a surface acoustic wave propagation velocity of 3488 m/sec and is piezoelectric. The system may have a single acoustic path or sets of acoustic paths which are, for example, parallel, as shown in FIG. 8A.
A wavefront produced by reflections from the leading and trailing edges of transducer fingers will be formed by the superposition of a first wave reflected from the first leading edge and successive waves reflected from successive edges and having differences in phase, with respect to the first wave, of -.lambda./4,.lambda./2,-3.lambda./4,.lambda., etc. As may be seen, the wave components having a phase -.lambda./4,.lambda./2 and -3.lambda./4 effect a cancellation, or at least an attenuation of the wave component reflected from the leading edge. The interdigital fingers of the transducers may therefore be advantageously split to reduce reflections, Conventional interdigital finger transducers which are constructed to operate at a fundamental, resonant frequency of 915 MHz, have a finger width (.lambda./4) of approximately 1 micron; a size which approaches the resolution limit of certain photolithographic fabrication techniques (the selective removal of metallization by (1) exposure of photoresist through a mask and (2) subsequent etching of the metallized surface to selectively remove the metal between and outside the transducer fingers). If the fingers are split, the width of each finger (.lambda./8) for a fundamental frequency of 915 MHz would be approximately 0.5 micron. The size would require sophisticated photolithographic fabrication techniques. In order to increase the feature sizes, the transducers in the transponder are constructed with a resonant frequency f.sub.0 of 305 MHz. In this case, the width of each finger is three times larger than transducer fingers designed to operate at 915 MHz, so that the width (.lambda./8) of the split fingers is approximately 1.5 microns. This is well within the capability of typical photolithographic fabrication techniques. Although such transducers are constructed with a resonant frequency of 305 MHz, they are nevertheless driven at the interrogation frequency of approximately 915 MHz; i.e., a frequency 3f.sub.0 which is the third harmonic of 305 MHz. The energy converted by a transducer, when driven in its third harmonic 3f.sub.0 (915 MHz), is about 1/3 of the energy that would be converted if the transducer were driven at its fundamental frequency f.sub.0 (305 MHz). Accordingly, it is necessary to construct the transducers to be as efficient as possible within the constraints imposed by the system. As is well known, it is possible to increase the percentage of energy converted, from electrical energy to SAW energy and vice versa, by increasing the number of fingers in a transducer. In particular, the converted signal amplitude is increased by about 2% for each pair of transducer fingers (either conventional fingers or split fingers) so that, for 20 finger pairs for example, the amplitude of the converted signal will be about 40% of the original signal amplitude. Such an amplitude percentage would be equivalent to an energy conversion of about 16%. In other words, the energy converted will be about 8 db down from the supplied energy.
The edge portions of the delay pads, as well as the lateral edges of the bus bars and (i.e., the edges transverse to the SAW paths of travel) are advantageously provided with two levels of serrations, which substantially reduce SAW reflections from these edges. The serrations include, for example, two superimposed "square waves" having the same pulse height but different pulse periods. For example, the pulse height for both square waves is .lambda./4, and the pulse period is .lambda./3 for one square wave and 6.lambda. for the other, where .lambda. is the SAW wavelength at 915 MHz. The first level of serrations serves to reduce reflections, while the second level serves to break up the average reflection plane.
The addition of finger pairs to the transducers therefore advantageously increases the energy coupling between electrical energy and SAW energy. However, as explained above in connection with FIGS. 1 and 3, the system according to the invention operates to excite the transducers over a range of frequencies between 905 MHz and 925 MHz. This requires the transducers to operate over a 20-25 MHz bandwidth: a requirement which imposes a constraint upon the number of transducer finger pairs because the bandwidth of a transducer is inversely proportional to its physical width. This relationship arises from the fact that the bandwidth is proportional to 1/.tau., where .tau. is the SAW propagation time from one side of the transducer to the other (the delay time across the transducer).
The transducer may be divided into several separate sections: a central section, two flanking sections and two outer sections. The central section includes interdigital transducer fingers which are alternately connected to two outer bus bars and to a central electrical conductor. This central section comprises a sufficient number of finger pairs to convert a substantial percentage of electrical energy into SAW energy and vice versa. By way of example and not limitation, there may be 12 finger pairs so that the converted amplitude is approximately 24% of the incoming signal amplitude. Flanking the central section, on both sides, are sections containing "dummy" fingers; that is, fingers which are connected to one electrode only and therefore serve neither as transducers nor reflectors. The purpose of these fingers is to increase the width of the transducer so that the outer sections will be spaced a prescribed distance, or SAW delay time, from the central section. For example, there may be 7 dummy fingers (or, more particularly, split fingers) in each of the sections. Finally, each of the outer sections of the transducer contains a single transducer finger pair which is used to shape the bandwidth of the transducer, e.g., maintain an effective bandwidth of about 25 MHz.
The transducer system preferably has an electrical impedance at the design frequency which matches the impedance of the antenna coupling, to maximize the power transfer between the antenna system and transducer. This matching is accomplished by forming series connections of transducer structures, which present as capacitive loads, to reduce impedance, as necessary, and providing heavy metal traces for the bus bars to reduce Ohmic losses. The bus bars are, for example, made approximately twice as thick as the other metallized elements on the substrate.
In practice, the metallization is deposited on the substrate surface using a two-layer photolithographic process. Two separate reticles are used in forming the photolithographic image: one reticle for the transducers, reflectors and phase pads as well as the alignment marks on the substrate, and a separate reticle for the bus bars. The process thus comprises the steps of depositing a 300 Angstrom layer of chromium and then 1000 Angstrom layer of aluminum on the substrate, followed by UV-exposure solubleizing resin spin coating, masking and etching of the bus bars, followed by deposition of 1000 Angstroms of aluminum and another UV activated resin spin coating, masking and etching to form the transducers, reflectors and phase pads as well as the alignment marks on the substrate, doubling the thickness of the bus bar structures.
Each two successive fingers of a transducer may be shorted at one or more locations between the bus bars. The shorts between successive fingers reduce energy loss due to Ohmic resistance of the fingers and render the reflector less susceptible to fabrication errors.
These various techniques and systems, described above, may advantageously be employed or combined with aspects of the present invention in known manner to achieve desired results.